Antireflection (AR) coatings on transparent articles reduce the reflectance of visible light from the articles and enhance the transmission of such light into, or through, the articles. When the articles are used as cover plates for display instruments, these coatings enhance the brightness, contrast, and readability of the displayed information, for a variety of lighting conditions. Optical articles such as ophthalmic lenses frequently are coated with antireflective coatings to decrease the level of reflected light and thereby increase visibility and minimize eye fatigue.
Anti-reflection (AR) coatings of various types and designs are well-known and used in a variety of applications. Some types of AR coatings comprise a single layer with refractive index lower than that of the substrate, or a stack of layers having alternating high and low refractive indexes. These coatings, while effective at reducing reflectivity, do not provide satisfactory durability for particularly harsh mechanical stress such as scratching or abrading, exposure to harsh chemicals such as acids or bases, and environmental conditions of temperature, humidity, sunshine or UV light exposure.
Some anti-reflection coatings comprise a single coating layer or film of material having an index of refraction lower than that of the transparent substrate material. Theoretically, a single layer coating having a refractive index equal to the square root of the refractive index of the substrate provides zero reflectance at a wavelength of light equal to four times the coating thickness. Magnesium fluoride (MgF2) and fluoropolymers are common single-layer coating materials, as their relatively low refractive indexes (<1.40) can provide reasonable anti-reflective performance on a glass or polymer substrate having refractive index greater than about 1.50. Silicon dioxide (silica) is also used as a single-layer AR coating, although it provides only moderate anti-reflective performance, because its refractive index (about 1.46) tends to be closer to that of common transparent substrate materials.
Other anti-reflection coatings comprise a multi-layer stack having carefully controlled, alternating high and low relative refractive indexes, which when combined with prescribed thicknesses of each layer, results in destructive interference of reflected light and significantly greater reduction in reflectivity over the visible wavelength range of 400 to 700 nanometers (nm). Coatings of this kind may comprise, for example, alternating layers of titanium dioxide (titania) as the high refractive index layers, and silica as the low refractive index layers.
Some single-layer and multi-layer AR coatings contain nano-particles (such as silica) in one or more coating layers, which can improve anti-abrasion and anti-glare properties. These nano-particles, however, tend to increase the overall reflectance of the coated article across the entire visible spectrum, while creating a matte finish surface appearance. This matte finish is unacceptable for certain applications such as ophthalmic lenses.
While various single-layer and multi-layer anti-reflection coatings have been generally effective in providing the desired optical properties, the coatings are not considered to have been entirely satisfactory for use in many applications. For example, some of the coatings show low resistance to mechanical damage, either through applied abrasive force, which causes scratches in the coating or removal of coating from the surface, or through stresses caused by mismatched thermal expansion coefficients of coating and substrate materials, which causes micro-cracking or crazing of the coating. Other coatings, particularly those for which all the layers are deposited by techniques such as electron beam deposition, reactive plasma sputtering, and ion-assisted deposition, show low durability upon prolonged chemical or environmental exposure, which makes them less than ideal for long-term consumer use, for example, in ophthalmic lens applications.
It should, therefore, be appreciated that there is a need for improved antireflection coatings and for an improved process for depositing such coatings onto substrates such as transparent polymeric and glass panels, and polymeric ophthalmic lenses, wherein the deposited coatings provide a satisfactory combination of low reflectance in the visible wavelength range, and high levels of mechanical, chemical, and environmental durability. The present invention fulfills this need and provides further related advantages.